The main fortress – known as 'the Citadel' – is built using quarrystone and it is a polygonal rampart fortress, whose irregular shape is due to its adept adaptation into a modern bastion fortress adjusted to the site's terrain. The conditions necessitated ordinary outworks on the field to the east, while the steep slope nearest the town served as a refuge area for the inhabitants of the town (Borgerskansen). Gyldenløve, Stortårnet and Overberget are free-standing forts to the east and south.
The plans for the fortress were designed by engineering officer Willem Coucheron on the order of the newly-appointed commanding general of Norway, Claus von Ahlefeldt.
The buildings at Fredriksten mainly comprise buildings erected or rebuilt after a big fire in 1826. These buildings, which particularly dominate 'the Citadel', are of a strict military empire style and are regarded as one of the best examples of this style in Norway. It covers a total area of about 600,000 square metres – approximately 150 acres.
Halden was attacked during six different Swedish campaigns; three from 1658 to1660, and three after the completion of the new fortress. The most well-known event at the fortress is the siege of 1718 when the Swedish King Charles XII was killed here. Fredriksten also played a major national role in the Swedish-Norwegian War of 1814 and the peaceful dissolution of the union with Sweden in 1905. It was decommissioned as an operational fortress in 1905.
After 1905, Fredriksten served as the headquarters of several military schools and staff units. The last school closed in 2005. The fortress is now an important regional cultural arena and tourist destination.